1. Field of the Invention
The invention in general relates to fluid detectors that can sense the difference between polar and non-polar fluids, and in particular to such a detector which remains accurate when conventional electric power lines come into contact with the polar fluid it is sensing.
2. Description of the Prior Art
The contamination of ground water by pollutants has become a problem in recent years. One type of such contamination occurs when storage tanks containing hydrocarbons, such as gasoline or oil, leak. Since many underground storage tanks that were installed during a "gas station boom" that occurred many years ago are now beginning to leak, and the cleanup of leaks that go undetected can easily run into millions of dollars, the detection of such leaks has become very important. Hydrocarbons, such as gasoline are non-polar fluids, while ground water is a polar fluid, and thus detectors that can sense the difference between polar and non-polar fluids have become a common means for detecting such gasoline leaks.
One type of detector that has been developed to detect hydrocarbon pollutants in ground water is one that employs two conductive probe members to which an oscillating voltage is applied. These detectors sense the differing conductivity of ground water and hydrocarbons to produce signals indicating which of the two fluids are present. See, for example, U.S. patent application Ser. No. 06/579,431, (now U.S. Pat. No. 4,586,033).
A problem that can occur with such detectors is that voltages and currents in the ground water in which the probe members are immersed can affect the measurements of the device and cause hydrocarbon alarms when no hydrocarbon is present. This problem has been found to occur with surprising frequency in areas, such as Florida, where the ground water levels are high. It has been found that the high ground water levels often result in the water coming in contact with power lines. ("Power Line" herein is defined as a conventional power line, such as those carrying 120 VAC, which generally carry power on the order of a hundred volts or greater.) Since areas with high water levels are also generally areas where underground tanks corrode and leak much more rapidly, it would be highly desirable to provide a detector which was not affected by the power lines coming in contact with the ground water.
The prior art detectors also have the problem that there is a large variation in resistance threshold, i.e. the probe resistance which causes a change in signal, from device to device. This requires careful resistance matching during manufacture which is highly labor-intensive and thus costly.